Process for the production of phosphoric acid at a high concentration and a gypsum by-product of improved quality

ABSTRACT

A process for the production of phosphoric acid at a high concentration and a gypsum by-product of improved quality which comprises the steps of mixing phosphate rock with phosphoric acid containing 36 to 44% P2O5, digesting the mixture with sulphuric acid, precipitating 70 to 80% of the calcium oxide in the phosphate rock as calcium sulphate hemihydrate in a first digester and precipitating all of the calcium oxide in the phosphate rock as calcium sulphate hemihydrate in a second digester, then filtering a slurry of calcium sulphate hemihydrate and washing the cake with a dilute acid, thereby producing phosphoric acid with a P2O5 concentration greater than 45 percent and phosphoric acid for digestion, and recrystallizing the calcium sulphate hemihydrate into calcium sulphate dihydrate in a mixed acid containing from 10 to 15 percent P2O5 and from 10 to 15% H2SO4 at a temperature of 50* to 80* C. and at a solids concentration of 20 to 45 percent, and then filtering and washing it, thereby producing a gypsum by-product containing less than 0.2% P2O5.

United States Patent Ishihara et a1.

[451 Apr. 4, 1972 [72] Inventors: Toshio Ishihara; Takayoshi Okazaki;Tetsuzo Endo; Tanizawa Koichi; Sataro Nakajima, all of Tokyo, Japan [73]Assignee: Nissan Kagaku Kogyo Kabushiki Kaisha,

Tokyo, Japan [22] Filed: Aug. 16, 1968 21 Appl. No.: 762,210

[30] Foreign Application Priority Data Sept. 21, 1967 Japan ..42/60253Mar. 11, 1968 Japan ..43/15554 [52] [1.8. CI ..23/165, 23/122 [51] Int.Cl. .,..C01b 25/22 [58] Field of Search ..23/165, 122

[5 6] References Cited UNITED STATES PATENTS 3,505,013 4/1970 Araki etal ..23/122 3,197,280 7/1965 Boyle et a1. ....23/l65 3,453,076 7/1969Long et a1 ....23/165 3,472,619 10/1969 Chelminski et al. ..23/165FOREIGN PATENTS OR APPLICATIONS 1,094,539 12/1967 Great Britain ..23/1656,800,867 7/1968 Netherlands ..23/165 6,612,150 3/1967 Netherlands.....23/165 Primary Examiner-Oscar R. Vertiz Assistant Examiner-GregoryA. Heller Attorney-Holcombe, Wetherill and Brisebois [5 7] ABSTRACT Aprocess for the production of phosphoric acid at a high concentrationand a gypsum by-product of improved quality which comprises the steps ofmixing phosphate rock with phosphoric acid containing 36 to 44% Pdigesting the mixture with sulphuric acid, precipitating 70 to 80% ofthe calcium oxide in the phosphate rock as calcium sulphate hemihydratein a first digester and precipitating all of the calcium oxide in thephosphate rock as calcium sulphate hemihydrate in a second digester,then filtering a slurry of calcium sulphate hemihydrate and washing thecake with a dilute acid, thereby producing phosphoric acid with a P 0concentration greater than 45 percent and phosphoric acid for digestion,and recrystallizing the calcium sulphate hemihydrate into calciumsulphate dihydrate in a mixed acid containing from to percent P 0 andfrom 10 to 15% H 80 at a temperature of 50 to 80 C. and at a solidsconcentration of to percent, and then filtering and washing it, therebyproducing a gypsum by-product containing less than 0.2% P 0 4 Claims, 1Drawing Figure Patented April 4, 1972 3,653,826

The present invention relates to a process for the production ofphosphoric acid at a concentration up to more than 40 percent P and agypsum by-product of improved quality, and more particularly to aprocess, highly suitable for industrial and economical applications, forproducing phosphoric acid at high concentration with high recovery and agypsum byproduct of improved quality, which comprises the steps ofdigesting phosphate rock with phosphoric acid and sulphuric acid underspecial conditions, thereby forming a slurry containing stableagglomerates of calcium sulphate hemihydrate crystals, fully washing itin the filtering stage, then repulping the calcium sulphate hemihydratein a mixed acid of extremely low concentration, re-crystallizing it intocalcium sulphate dihydrate, and finally filtering and washing it withwater.

In the conventional wet process for producing phosphoric acid, an acidat a low concentration of about 30% P 0 is usually produced and thisacid is evaporated to a higher concentration for practical application.Numerous attempts have been made to obtain phosphoric acid at a higherconcentration directly, without subsequent evaporation, but none of themhas proved practically successful.

For instance, S. Nordengren has proposed in his US. Pat. No. 1,776,595 amethod of producing high-concentration phosphoric acid after separatingcalcium sulphate in the form of anhydrite or hemihydrate dependent onthe relation among acid concentration, temperature and form of calciumsulphate. However, when the concentration of phosphoric acid is high,the digestion of phosphate rock is difficult; moreover, the crystals ofcalcium sulphate hemihydrate are finer and less easy to separate thanthose of calcium sulphate dihydrate, and they are unstable, being liableto recrystallize into calcium sulphate dihydrate during the filtrationstage. Thus, at present, this method has not been commerciallysuccessful.

Studies and inventions have been made in an attempt to eliminate thesedrawbacks.

For instance, US. Pat. No. 2,885,264 discloses a method of adding to thefirst tank 1.5-3 percent less sulphuric acid than a stoichiometricequivalent to the calcium oxide in phosphate rock, thereby precipitating85-90 percent of the monocalcium phosphate into calcium sulphatehemihydrate, and precipitating all of the remaining monocalciumphosphate into calcium sulphate hemihydrate in the second tank for thepurpose of obtaining easily filterable crystals of calcium sulphatehemihydrate.

Belgian Pat. No. 676,166 proposes a method of producing a slurrycontaining an easily filterable and washable calcium sulphatehemihydrate which comprises the steps of precipitating -60 percent ofcalcium oxide into calcium sulphate hemihydrate in the first tank withcirculation of a large quantity of hot slurry, and in the second tankprecipitating all the rest as calcium sulphate hemihydrate. In addition,I & EC Process Design and Development, Vol. 4, page 85, AmericanChemical Society, Washington, D. C., 1965, describes a process ofspraying sulphuric acid on the foams covering the slurry surface andthereby controlling the decomposition, instead of directly reactingsulphuric acid with the slurry.

Even in these methods, however, the apparatus does not permit the easycirculation of hot slurry in a large quantity, and the resulting calciumsulphate hemihydrate is of low purity. By reason of these drawbacks,these methods have not been found satisfactory for industrial use.

On the other hand, there is method which eliminates the step offiltering and washing calcium sulphate hemihydrate and simply separatescrystals from the slurry containing calcium sulphate hemihydrate andwithout washing, repulps and recrystallizes them in the next stage andthereby produces calcium sulphate anhydrite, as disclosed in US. Pat.No. 2,531,977; or produces calcium sulphate dihydrate, as described inChemical Engineering Progress, Vol. 62, page 108, American Institute ofChemical Engineers, Philadelphia- New York, 1966. In both methods it isproposed to wash calcium sulphate in the form of stable anhydrite ordihydrate.

Certainly this is one of the effective means to avoid washing thecalcium sulphate produced in the form of hemihydrate; but once thecalcium sulphate is dehydrated into calcium sulphate anhydrite, it wouldbe unsuitable for use as board or cement. On the contrary, when calciumsulphate hemihydrate is separated without washing, and recrystallizedinto calcium sulphate dihydrate, the P 0 concentration during therecrystallizing stage will necessarily be increased due to the absenceof washing. Moreover, if in the digestion stage the phosphoric acidproduced is to be obtained at higher concentration, the P 0concentration will be increased by that much and in consequence, saidconcentration in the recrystallizing stage will in practice become closeto the conventional value of about 30 percent.'Thus, the hydration ofcalcium sulphate takes an extremely long time and yet the formeddihydrate will contain relatively great quantities of impurities such asphosphates and fluorine compounds.

In the above-mentioned techniques, usually in order to obtain phosphoricacid at a high concentration, the method of separating calcium sulphateeither in the form of hemihydrate or anhydrite is adopted. As thecrystals of these compounds are finer than those of calcium sulphatedihydrate, they are very hard to filter and wash. Calcium sulphatehemihydrate is particularly unstable and during filtration and washingit slowly recrystallizes into calcium sulphate dihydrate, tending tocause various difficulties. Accordingly, a high recovery cannot beexpected.

When the problem is viewed from the standpoint of the gypsum by-productwe see that even in the commercial process utilizing calcium sulphatedihydrate from which the low-concentration phosphoric acid of 30 percentP 0 is obtained, small amounts of phosphoric acid and phosphates remainin the gypsum and it is known that the presence of phosphoric acid andphosphate in gypsum is undesirable when that gypsum is to be used ingypsum board or Portland cement. For this reason, the present methodsare to rewash the gypsum to reduce the amount of soluble phosphate onthe surface of the crystals or, to do what is called, improvement, i.e.,calcine and completely neutralize the water-soluble phosphoric acid inthe gypsum before use. In view of this content of phosphoric acid andphosphates, even in the gypsum by-product resulting from the manufactureof low-concentration phosphoric acid, ti is easily inferable that stillmore impurities will be contained in the gypsum by-product from themanufacture of high-concentration phosphoric acid. In the sense ofeffective utilization of phosphate rock, however, the use of the gypsumby-product resulting from the manufacture of phosphoric acid is aproblem calling for serious attention. The technique of manufacturingphosphoric acid should be developed by balancing the requirements ofthese two resulting products.

Specifically, the primary consideration in the manufacture ofhigh-concentration phosphoric acid will be how to easily separate thegypsum by-product from phosphoric acid and the second will be how highto maintain the purity of the gypsum. Moreover, the apparatus must beindustrially simple and trouble-free, and, of course, economical with agood liquid balance as a complete system.

In view of this situation, the present inventors have conducted studieson the industrial production of high-concentration phosphoric acid andmade the following discovery.

The calcium sulphate hemihydrate formed in a phosphoric acid solutioncontaining a concentration of 45-55% P 0 contains considerablequantities of phosphoric acid, although the quantities differ independence on the digesting condition. In the first stage of digestionwith an excess sulphuric acid, phosphate rock is not digestedcompletely; but when no free sulphuric acid is present, much monocalciumphosphate is formed. Meanwhile it is recognized that, when agglomeratecrystals which look easy to filter are formed, these crystals of calciumsulfate hemihydrate contain more phosphoric acid and phosphate than isthe case in non-agglomerate crystals. Most of the phosphoric acidcontained in these agglomerate crystals of calcium sulphate hemihydratedoes not exist on the surface of crystals, but is inside the crystalsand does not easily dissolve out with washing. However, the phosphateentrapped within said agglomerate crystals of calcium sulphatehemihydrate is soluble.

It should be noted that this calcium sulphate hemihydrate containing asubstantial quantity of soluble phosphates is comparatively stable towater; and its hydration in the stage of filtering and washing in muchslower than that of calcium sulphate hemihydrate containing a smallerquantity of soluble phosphates. The crystalline structure of calciumsulphate and other factors may be involved in this phenomenon, but it iseasily predictable from common observation that hydration is retarded inphosphoric acid solutions. It has also been found that the quantity ofphosphoric acid or phosphates can be controlled in dependence on thedigesting condition of phosphate rock.

In general, if the aim is to obtain a gypsum by-product of improvedquality as well as phosphoric acid at a'high concentration with highrecovery, washing while in the form of calcium sulphate hemihydrateshould, if possible, be avoided, with only a portion of the phosphoricacid solution being separated, and hydration should desirably take placein the next stage when there is a lowered concentration of phosphoricacid, and full washing should be carried out when the material is in theform of calcium sulphate dihydrate. ln this case, however, since all thefiltrate in the system after filtration of recrystallized calciumsulphate dihydrate should be used up, it is impossible for reasons ofliquid balance to determine the concentration of phosphoric acid forrecrystallization arbitrarily. When the concentration of the phosphoricacid produced is to range between 45 and 50 percent, the concentrationof phosphoric acid in the solution to be recrystallized will be 27-30percent. This is equivalent to the P concentration of the conventionaldihydrate process, in which recrystallization will take a very long timeand it will be impossible to eliminate the impurities in the gypsum,i.e., phosphates and fluorine compounds, any better than by theconventional dihydrate process.

The present invention is free from these drawbacks. After variousstudies, the present inventors have developed the present invention,based on the'following process: First, the phosphate rock is mixed inpre-mixer with a solution containing 36-44% P O to be provided from alater step, and formed into a slurry. Next, in the first digester,sulphuric acid is added, precipitating 70-80 percent of calcium oxide inthe rock into calcium sulphate hemihydrate and 20-30 percent of it intomonocalcium phosphate. Then, in the second digester, the monocalciumphosphate is precipitated as calcium sulphate hemihydrate whileconstantly maintaining the concentration of free sulphuric acid at 2-3percent. Then, a portion of said slurry is recirculated for use in thepre-mixer or in the first digester, the rest of said slurry beingfiltered, and phosphoric acid separated therefrom. The cake of calciumsulphate hemihydrate is washed with the filtrate of calcium sulphatedihydrate to be provided from a later step, said filtrate being re-usedfor the digestion of phosphate rock. The cake is repulped to a 20-45percent solids concentration, at a temperature of 50-80 C. in a diluteacid containing -15% P 0 and l0-l5percent sulphuric acid, and shortlyrecrystallized into calcium sulphate dihydrate. It is then filtered andwashed to separate the calcium sulphate dihydrate and thereafter, thefiltrate is utilized as a washing solution for calcium sulphatehemihydrate, while the washing filtrate is recirculated for use in therepulping of calcium sulphate hemihydrate. Thus, a high-concentrationphosphoric acid can be produced with an improved quality of gypsum as abyproduct.

In this specification, the term high-concentration phosphoric acid isdefined as a phosphoric acid having a P 0 concentration as high asapproximately 40-55 percent and a free sulphuric acid concentration ashigh as 2-3 percent.

The method of this invention is advantageous for producing ahigh-concentration phosphoric acid having a 45-55% P105 concentration,but if the liquid balance is properly taken into account, the method mayalso be used to produce a phosphoric acid having a concentration ofabout 40% P,O,.

In the definition of this invention, the term gypsum byproduct ofimproved quality means a gypsum in which the total P 0 contained in thecalcium sulphate dihydrate is less than 0.2 percent, with extremelysmall amounts of such impurities as fluorine compounds, and in the formof desirable crystals regardless of the source of phosphate rock.

The important point in the method of this invention, which involves thewashing of calcium sulphate hemihydrate, is how to obtain calciumsulphate hemihydrate which contains no non-digested phosphate rock butdoes contain a certain amount of soluble phosphoric acid and phosphate.This constitutes a vital element of invention, in combination with theproportions of split additions of sulphuric acid in the digestionprocess. Stable, agglomerate calcium sulphate hemihydrate can be formedby firstly precipitating 70-80 percent of the calcium oxide in phosphaterock as calcium sulphate hemihydrate and 20-30 percent of it asmonocalcium phosphate by digesting the phosphate rock with sulphuricacid and phosphoric acid, and then precipitating the monocalciumphosphate into calcium sulphate hemihydrate while 2-3 percent freesulphuric acid is constantly retained.

The calcium sulphate hemihydrate formed in this manner is very stable towashing. It is desirable to use a sulphuric acid of about 95-985 percentconcentration in this process.

in the first stage of acid digestion, if less than 70 percent of thecalcium oxide in the rock is precipitated as calcium sulphatehemihydrate and more than 30 percent of it as monocalcium phosphate, theformed agglomerate crystals will be adapted to be filtered out, but thedigestion of phosphate rock is not complete, which results in pooryields.

If, by contrast, more than percent of the calcium oxide is precipitatedas calcium sulphate hemihydrate and less than 20 percent of it asmonocalcium phosphate, the digestion ratio of the rock may be improved,but the agglomerate crystals become rather small, which makes them moredifficult to filter them out. From the standpoint of thermal balance,too, the split ratio of sulphuric acid introduction in this invention isfound very easy to control.

Next, in the second stage of acid digestion, in which sulphuric acid isadded to convert the monocalcium phosphate to calcium sulphatehemihydrate and phosphoric acid, filtration will be difficult if thereis less than 2 percent free sulphuric acid. But, if there is more than 3percent, the process will not be economical.

The preferred reaction temperature range of this process is Z0 -80 C. inthe pro-mixer and l00 C. in first and second digesters. The slurrycontainirig the calcium sulphate hemihydrate formed should desirably becooled to about 70 C. in the receiver tank and filtered at thistemperature.

The preferred reaction period is 5-10 minutes in the premixer, and aboutl-2.5 hours in the first and second digesters,

under agitation.

The preferred proportion between the amount of calcium sulphatehemihydrate slurry to be returned from the slurry distributor torecirculate to the pre-mixer or to the first digester and the amount ofslurry to be filtered by the calcium sulphate hemihydrate filter isbetween 1/1 and 2/1 from the standpoint of thermal balance. Thetemperature should be about 70 C.

The liquid for hydrating the calcium sulphate hemihydrate obtained fromthe digestion stage should contain 10-1 5% P 0 and l0-l5 percentsulphuric acid, the temperature being 501-80 C. and preferably 60-79: Cand thg solids congen;

tration should lie betwe eri 2m45 percent and preferably 30-40 percent.Under these conditions hydration is completed in about minutes.Moreover, calcium sulphate dihydrate in excellent crystal formcontaining less than 0.2Y% P 0 can be produced. From a mixed solution ofphosphoric acid and sulphuric acid with a concentration of l-l5% P 0 and10-15 percent sulphuric acid, good crystals of gypsum can be produced ina short time if the temperature, solids concentration, and other factorsmatch. When the P 0 concentration in 5 this mixed acid is below percent,calcium sulphate dihydrate will precipitate very rapidly, and iscompleted in less than 30 minutes, but the crystals become ratherneedleshaped, which is undesirable. If the P 0 concentration is morethan percent, hydration is slow, greatly increasing the proportion ofphosphoric acid and phosphates in the calcium sulphate dihydrate, whichis undesirable. Meanwhile, if the sulphuric acid concentration in thismixed acid is less than 10 percent hydration will be slow and theproportion of phosphoric acid and phosphates in the gypsum will beincreased. By contrast, when the sulphuric acid concentration is over 15percent, the temperature for hydration will be difficult to control, andinvolves the possibility of dehydration to calcium sulphate anhydrite.

When the sulphuric acid concentration in the hydration liquid, i.e., thewashing filtrate from the calcium sulphate dihydrate filter, is low whenrepulping the calcium sulphate hemihydrate, more sulphuric acid is addedto make the conmentioned factors matched, calcium sulphate dihydrate ofimproved quality with extremely small quantities of phosphoric acid,phosphates and impurities such as fluorine compounds can be produced.

The calcium sulphate hemihydrate slurry obtained by digestion isfiltered, separating the phosphoric acid product containing 45-55% P 0Moreover, on account of the abovementioned effect of the proportionsinto which the sulphuric acid added at various times in the digestionstage is split, this calcium sulphate hemihydrate is washable. Thereforeit is washed with the filtrate from the calcium sulphate dihydratefiltration obtained in a subsequent stage and then sent to therecrystallizer for repulping. As a result the P 0, concentration I inthe solution in the recrystallizer can be limited to lO-l5 1 percentagainst 30 percent in a process comprising no washing step. Thus, byadding sulphuric acid to the solution and conducting the hydration at a10-15 percent H,SO concentration, it becomes possible to produce calciumsulphate dihydrate having well shaped crystals with a total P 0 contentless than 0.2 percent and an extremely small amount of impurities suchas fluorine compounds, regardless of the source of phosphate rock.

Details of its method of this invention will now be centration equal to10-15 percent. The hydration temperature described with reference to theattached drawing, which is a should be maintained at 50-80 C. If thetemperature drops below 50 C., hydration may be completed within thespecified time of 120 minutes in this invention, but the proportion ofphosphoric acid and phosphates in the formed gypsum flow sheetexplaining said method.

As shown in this drawing, the ground phosphate rock isv weighed andcharged into a pre-mixer 2 via a pipe 1.

Phosphoric acid is added to the pre-mixer 2 through a pipe 15 will begreatly increased. By contrast, at over 80 C.,hydration for digestion.The acid contains 36-44 percent P 0, and is will be incomplete.

The present inventors have discovered that the solids concentrationduring hydration has great influence on the phosphoric acid andphosphates contents of gypsum. Specifiprepared from the filtrate fromthe second section of a filter l7 and from phosphoric acid produced bythe process. The ground rock is thoroughly mixed and distributed. Thetemperature is thereby held at 70-80 C. and the retention time is cally,to obtain good gypsum according to this invention, the 5-10 minutes.Then the slurry, which has been partially turned solids concentrationshould be 20-45 percent. If hydration is carried out at less than 20percent solids concentration, the crystalline form of the gypsum and thehydration time may be satisfactory, but the P 0 content of the gypsumwill be increased, producing gypsum of less desirable quality thanexpected. This is presumably because hydration is materially affected bythe solids concentration. As this concentration decreases, the hydrationspeed increases, which affects the entrapment of phosphoric acid intothe gypsum. On the contrary, a solids concentration exceeding percentwill make agitation practically impossible.

The high stability to washing of the calcium sulphate hemihydrateobtained through acid digestion according to the method of thisinvention will be apparent from following test example.

Test

Thirty grams of calcium sulphate hemihydrates obtained by threedifferent processes was suspended in 100 g. of water,

1 into monocalcium phosphate, overflows into the first digester 3, towhich at the same time 98 percent sulphuric acid is supplied through apipe 13 branching off a pipe 12. Meanwhile,

the slurry circulated from a receiver tank 5 is delivered by a pump 8 toa slurry distributor 9 where said slurry is divided and a portionsupplied to the digester 3 through a pipe 11. The quantity of sulphuricacid added, together with the amount contained in the circulated slurryto be added to the first digester 3, should be sufficient to precipitate7080 percent of the calcium oxide in the rock as calcium sulphatehemihydrate, the temperature being 90l00 C. and the retention time aboutl-2.5 hours. (The circulated slurry from the receiver tank 5 may insteadbe added to the pre-mixer 2.) 1 Then the slurry overflows into thesecond digester 4. Sulfuric acid is added to the digester 4 through apipe 14 branching off the pipe 12, thereby maintaining the concentrationof free sulphuric acid constant at 2-3 percent. In this digester all themonocalcium phosphate in the slurry is precipitated as calcistirred atC., and at definite intervals of time the .quantity um sulphatehemihydrate, the temperature in said second of water of crystallizationwas measured. The results are given IahleL As shown by this example, thecalcium sulphate hemihydrate formed according to the method of thisinvention is stable to water and is fully washable in the stage offiltering and washing, and accordingly the succeeding stages ofrepulping and recrystallization can be carried out with an extremely lowconcentration of phosphoric acid. Recrystallization can be completed ina very short time. further, all the abm idigester being 90- l the firstdigester 3. v

The slurry in the digester 4 goes to the receiver tank 5, from which itis delivered by a pump 6 to a cooler 7, where it is cooled to about C.The cooling method is not critical. Any

60 routine method will suffice, but air cooling is desirable. The

cooled slurry is delivered by the pump 8 to the slurry distributor 9,where it is divided, one portion passing through a pipe 10 to the filter17 for calcium sulphate hemihydrate and the other portion circulatingthrough the pipe 11 to the first The amount circulated is preferably 1to 2 times the amount of slurry supplied to the filter 17. This servesfor temperature y mr d acts sess- H n. 1 The phosphoric acid filtered bythe filter 17 has a 45-50% P 0 concentration and a 2-3% H concentration.Part of it is employed for controlling the concentration of the acid fordigestion and the rest passes trough a pipe 16 to the storage tank.Meanwhile, the cake of calcium sulphate hemihydrate I from which thephosphoric acid has been separated is washed in the second section ofthe calcium sulphate hemihydrate filter 17 with the filtrate from thefirst section of the calcium sulphate dihydrate filter 19, said filtratebeing supplied via a pipe 22. As described above, this washing filtrate.passes through the pipe 15 and, after adjustment with phosphoric acid,is sent to the pre-mixer 2, there to be used.

The cake thus washed is moved to a recrystallizer 18, where it isrepulped by the washing filtrate supplied through the pipe 21 from thesecond section of the filter 19 for iahzirn sulphate dihydrate and thesolution is kept at a 10-15% 1 ,0, concentration, and 10-15% l-l,Sconcentration by adding sulphuric acid supplied through the pipe 25.Then, as the result of recrystallization taking place at the temperatureof 50-80 C. for 120 minutes in the recrystallizer 18, all the calciumsulphate hemihydrate recrystallizes to calcium sulphate in the calciumsulphate hemihydrate such as P 0 F, etc. are dissolved away and thecontent of P 0 fluorine and other impurities in the gypsum by-product isthereby minimized.

The slurry formed at this point travels through a pipe 20 to the firstsection of the filter 19, where it is filtered. The gypkg/hr. of asolution containing 14.5% l Q and 1 0% H;SO

thereby producing 24.3 kg./hr. of phosphoric acid containing 45% P 0 and2.5% 11,80 32 kg./hr. of washing filtrate containing 33.2% P 0 and 37.7kg./hr. of calcium sulphate hemihydrate cake. Using said washingfiltrate and a portion 0 the phosphoric acid produced, the acid formixing with the phosphate rock was prepared, and recirculated for use.

dihydrate. At the same time the various impurities contained: 1 The cakewas P into the recrystallizer, p p with kg./hr. of the washing filtrateof calcium sulphate dihydrate filter from the next stage, 1.12 kg./hr.of 98.5 percent sulphuric acid was then added so as to make a suspensionwith a 40 percent solids concentration, 14.5% P 0 concentration, and

sum by-product is further washed in the second filter section; withwater or hot water supplied through a pipe 26, the calcium sulphatedihydrate by-product being obtained at 27. As mentioned earlier, thefiltrate in the first section is supplied indicated 0.18 percent total 1,0 and 0.04

through the pipe 22 as the washing fluid for the calcium sulphatehemihydrate.

In this case, in order to more perfectly prevent conversion of calciumsulphate hemihydrate into calcium sulphate dihydrate in the filter dueto washing, it is possible to neutralize the sulphuric acid in thesolution with lime 'or ground phosphate rock in the neutralizing tank23. It goes without saying that neutralization will not always benecessary for calcium sulphate hemihydrate obtained through digestion inthis invention under conditions fully meeting the requirements of thisinvention.

The phosphoric acid produced by the method of this invention has aconcentration of P 0 as high as 40-55 percent with a high recovery ofover 98% P 0 The calcium sulphate hemihydrate formed by this inventionis so stable to water that it can be fully washed in the stage offiltering and washing. Accordingly the subsequent repulping EXAMPLE 1 Apre-mixer having an effective volume of 7.5 liters and two literdigesters were used. Florida phosphate rock containing 33.3% P 0 and47.4% CaO, and a phosphoric acid solution containing 36.1% P 0 and 4.2%H SO were fed at the rates of 18 kg./hr. and 43.3kg./hr. respectivelyinto the premixer and mixed together at 75 C. They were then permittedto overflow into the first digester, to which additions of 70 C.

circulated slurry at a rate of 104 kg./hr. and 98.5 percent sulphuricacid at a rate of 6.84 kg./hr. were made. Digestion continued for 1.2hours at 95 C. The feed rate of sulphuric acid required forprecipitating all the CaO in the rock as calcium sulphate hemihydratewas 14.9 kg./hr. The feed rate of sulphuric acid to the first digesterwas 1,82 kg./hr. from the pre-mixer and 1.78 kg./hr. in-the circulatedslurry, while the fresh addition of sulphuric acid was 6.73 kg./hr.(98.5 X 6.84 kg./hr.), totaling 10.33 kg./hr. Thus, the total amount ofsulphuric acid in the first digester corresponded to just percent of thestoichiometrically equivalent amount required for digestion.

The slurry overflowing out of the first digester passed to the seconddigester, where with further addition of 98.5 percent sulphuric acid ata rate of 7.18 kg./hr., reaction took place at 10% H SO concentration,and stirred at 70 C. for 2 hours. The resulting slurry was filtered at arate of 64.5 kg./hr., washed with 19.3 kg./hr. of water, and dried,producing 25.8 kg./hr. of dried calcium sulphate dihydrate, analysis ofwhich percent water soluble P 0 Calculated therefrom, the recovery of P0, was 99.3 percent. The filtrate from the calcium sulphate dihydratefiltration was utilized for washing the calcium sulphate 30 hemihydrateand the washing filtrate was piped to the recrystallizer, to be utilizedfor repulping.

EXAMPLE 2 Using the same apparatus as in Example 1, l8 kg./hr. of Kolaground phosphate rock (percent passing l00-mesh and 70percent passingZOO-mesh) containing 35.36 percent P 0 and 50.3% CaO, and 49.7 kg./hr.of phosphoric acid solution containing 43.8% P 0 5 and 3.16% H 50 werefed 40 into the pre-mixer, mixed at .75" C.; and overflowed into thefirst digester, in which digestion continued for 1.2 hours at 93 C. with77 kg./hr. of 70 circulated slurry and 9.95 kg./hr. of 98% H 80 beingadded. The quantities of sulphuric acid added to this digestercorresponded to 80 percent of the 45 stoichiometric equivalent forprecipitating all CaO in the rock as calcium sulphate hemihydrate.

The slurry overflowing out of the first digester went to the seconddigester, where an additional 6.85 kg./hr. of 98% l-l SO was added andthe reaction carried out at 93 C. for 1.2

50. hrs. The total quantities of H,SO were in excess of thestoichiometrically equivalent amount, the concentration of sulphuricacid in the solution of the second digester being constantly maintainedat about 2.5 percent.

The digested slurry was then cooled to 70 C., filtered'at a 55, rate of77 kg./hr., and immediately thereafter washed with 22.8 kg./hr. of asolution containing 12% P 0 and 10% H SO thereby producing 28.9 kg./hr.of phosphoric acid containing 52 percent P 0 and 2.5% 11,80 32.3 kg./hr.of washing filtrate and 36.5 kg./hr. of cake. Using approximately thesame process as described above, a phosphoric acid solution 1 containing54% P 0 concentration was obtained.

EXAMPLE 3 heated to 70 C., and maintained at this temperature whilebeing stirred. The reaction was completed after minutes, producingcalcium sulphate dihydrate, which was filtered,

75 washed and dried into 12.5 kg. of calcium sulphate dihydrate.

The reaction system after 120 minutes had a 40.7 percent solidsconcentration and the composition of the mixed acid included 10.5% P and9.37% H 80 The analysis of the calcium sulphate dihydrate produced wasas follows:

EXAMPLE 4 Calcium sulphate hemihydrate for testing was prepared in thesame manner as in Example 1. Said calcium sulphate hemihydrate (7.9percent water of crystallization, 1.02 percent total P 0 and 0.25percent water-soluble P 0 in the amount of 12.3 kg. was added to 24.1kg. of mixed acid containing 10.8% P 0 and 15% 11:50., which had beenheated to 50 C., and stirred with said temperature maintained. 60minutes later, the reaction was stopped, yielding a slurry of calciumsulphate dihydrate, which was filtered, washed and dried into 10.65 kg.of calcium sulphate dihydrate.

The reaction system after 60 minutes had a 29.4 percent solidsconcentration and the composition of mixed acid contained 10.6% P 0 and14.7% H 30 The analysis of the product was as follows:

Water of crystallization 19.5%

CaO 31.6%

Total 9,0, 0.18%

Water soluble 1*,0, 0.04%

Example 5 H 50, added to the Size of first digester P 0 in agglomerate H50 required (7%) Non-digested crystals crystal for digestion P,O (91:)(72) p,

From these results it was concluded that the filterability of calciumsulphate hemihydrate is best when a 50 percent{ precipitation of calciumsulphate occurs in the first digester, and the digestion ratio orstability of calciu n sulphate hemihydrate to washing is best with 95percent precipitation.

In the present invention, 70 percent precipitation has been adopted tosecure the most advantageous combination of both stability andfilterability.

What is claimed is:

1. In a process for producing phosphoric acid and gypsum containing lessthan 0.2% P 0 by digesting phosphate rock with sulphuric acid andphosphoric acid to produce a high concentration of phosphoric acid andcalcium sulphate hemihydrate, filtering the resulting mixture toseparate the phosphoric acid, washing, repulping and recrystallizingsaid calcium sulphate hemihydrate to produce calcium sulphate dihydrate,and filtering and washing the resulting calcium sulphate dihydrate toobtain by-product gypsum the improvement comprising step one pre-mixingphosphate rock with phosphoric acid derived from the subsequent stepsfour and five which has a P 0 content adjusted to 36-44 percent byweight, at 7080 C. to form a slurry, step two in a first digester addingsulphuric acid to the slurry to digest all of said phosphate rock byprecipitating from 70 to percent by weight of the calcium oxide in thephosphate rock in the form of calcium sulphate hemihydrate and todissolve the rest of the calcium oxide in the phosphate rock by formingmonocalcium phosphate, at 100 C., step three in a second digesterreciilpitating the monocalcium phosphate as calcium sulphate em ydrateby adding sufficient sulphuric acid to maintain the content of freesulphuric acid in the liquid phase of the slurry constant at 2 to 3percent by weight, at 90100 C., step four circulating a part of theresulting slurry to said step one before filtering while the ratio ofdividing of slurry to be filtered is 2l:l, filtering the remainder ofthe slurry thereby separating the product phosphoric acid which has a4055% P 0 content therefrom and recycling a part of the productphosphoric acid to said step one with the washings of calcium sulphatehemihydrate, step five washing the solid residue of calcium sulphatehemihydrate with the filtrate from step seven and circulating thewashings to said step one, step six repulping and recrystallizing saidwashed calcium sulphate hemihydrate into calcium sulphate dihydrate in amixture of phosphoric acid and sulphuric acid which contains morephosphoric acid than sulphuric acid obtained by adding sulphuric acid tothe washings of step eight, and of which the phosphoric acid andsulphuric acid content are both adjusted to 10-15 percent by weightrespectively, the acid mixture of step six contains a 20-40 percentsolids concentration and the recrystallizing step proceeds at atemperature between 50 and 80 C., step seven separating the calciumsulphate dihydrate by filtration and recycling the filtrate to stepfive, step eight washing the resulting calcium sulphate dihydrate withhot water and recycling the washings to step six.

2. The process of claim 1, in which part of the resulting slurry of stepthree is recycled to step two before filtering the remaind of theslurry.

3. The process of claim 1, in which said sulphuric acid added to steptwo has a concentration of about to 98.5%.

4. The process of claim 1, in which the reaction period in the step onepre-mixer is 5-10 minutes, and the reaction periods in the first andsecond digesters is l to 2.5 hours.

UNITED STATES PATENT OFFICE CERTIFICATE 9F CURREGTION Patent No.3,653,826 Dated April 4, 1972 I TOSHIO ISHIHARA, TAKAYOSHI OKAZAKI andTETSUZO ENDO It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

[22] Filed September 16, 1968 Signed and sealed this 9th day of January1973.

(SEAL) Attest:

EDWARD M FLETCHER ,JR ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM PO-IOSO (10-69) USCOMM-DC 6O376-F'69 u.s, GOVERNMENTPRINTING OFFICE: 1969 0-365-334

2. The process of claim 1, in which part of the resulting slurry of stepthree is recycled to step two before filtering the remaind of theslurry.
 3. The process of claim 1, in which said sulphuric acid added tostep two has a concentration of about 95 to 98.5%.
 4. The process ofclaim 1, in which the reaction period in the step one pre-mixer is 5-10minutes, and the reaction periods in the first and second digesters is 1to 2.5 hours.